I think what he wants is to introduce mandatory military training for kids coming out of school and then those who are trained up can be called upon first in the event of needing to defend the homeland within a few years after they complete their training and it means there will always be several hundred thousand members of society who recently completed some military training and can quickly be mobilized to defend the homeland.
This is what Finland does and it means they always have around 200k people who can be called up to defend the homeland without needing training because they already know how to use weapons and work in a group
That being said - I'm not sure who can invade Britain though
Nukes kind of make it pointless for us really.